Summary: Methylenetetrahydrofolate reductase (MTHFR) is an essential enzyme in folate metabolism and in DNA methylation and synthesis. The role of two common polymorphisms at the MTHFR gene, C677T and A1298C, in the etiology of childhood or adult acute lymphoblastic leukemia (ALL) has been previously investigated. Although a protective effect of MTHFR*677T against ALL was systematically reported, the magnitude of the effect appeared to be influenced by population-specific gene-environmental interactions. The evidence of the role of MTHFR*1298C in ALL susceptibility was less consistent, emphasizing the need for enlarging molecular epidemiologic studies to independent trials from different populations. The authors analyzed in North Portugal the association between variations at the two MTHFR positions and risk of ALL by comparing genotypes and gene frequencies in 103 affected children with those in 111 healthy controls. None of the variations was found to significantly affect the risk of developing childhood ALL in North Portugal, and this finding per se is of relevance in further studies aimed at assessing the etiology of the pathology in this specific population. Despite the absence of statistical significance, these data revealed that the frequency of MTHFR*677T was lower in patients than in controls, a result that is congruent with other reports and with the functional model usually invoked to explain its ALL protective effect. Concerning MTHFR1298*C, this study failed to corroborate previous findings of decreased risk of ALL in the presence of the variant.